Fuel cells are electrochemical devices that convert chemical potential energy into usable electricity and heat without combustion as an intermediate step. Fuel cells are similar to batteries in that both produce a DC current by using an electrochemical process. A fuel cell has two electrodes, an anode and a cathode, that are separated by an electrolyte. Like batteries, fuel cells are combined into groups, called stacks, to obtain a usable voltage and power output. Unlike batteries, however, fuel cells do not release energy stored in the cell, running down when stored energy is gone. Instead, they convert energy from a hydrogen-rich fuel directly into electricity and operate as long as they are supplied with the fuel and oxidant. Fuel cell power systems emit almost none of the carbon monoxide or nitrogen compounds released by conventional combustion of gasoline or diesel fuel, and can utilize a wide variety of fuels: natural gas, coal-derived gas, landfill gas, biogas, alcohols, gasoline, or diesel fuel oil. Accordingly, there is a desire to employ fuel cells to power motor vehicles as a way of reducing noxious emissions.
Methods for generating hydrogen-rich fuel for a fuel cell from petroleum-based hydrocarbons include steam reforming and partial oxidation. Between these two methods, steam reforming is disadvantageous because it requires a sizeable apparatus that requires a heat source to maintain its operation, whereas partial oxidation can be achieved in a catalytic process that utilizes a smaller reactor size than steam reforming. In addition, a catalytic partial oxidation apparatus on an automotive vehicle typically attains its operation more quickly after the engine is started than does a vehicle-based steam reforming apparatus. However, catalytic partial oxidation reactions typically occur at about 900° C. to about 1300° C., at which temperatures the catalytic activity often degrades.
The Solid Oxide Fuel Cell (SOFC) is an example of a technology that can utilize a partial oxidation catalyst. SOFC is considered to be the most desirable fuel cell for generating electricity from hydrocarbon fuels. This is because it is simple, highly efficient, tolerant to some impurities, and can internally reform low concentrations, less than 1%, of hydrocarbons. One of the big advantages of the SOFC over some competing technologies is that the electrolyte is a solid. This means that no pumps are required to circulate hot electrolyte. Another big advantage of the SOFC is that both hydrogen and carbon monoxide are used in the cell. This means that the SOFC can readily and safely use many common hydrocarbons fuels such as natural gas, diesel, gasoline, alcohol and coal gas.
SOFC does not require an expensive liquid cooling system. In fact insulation may be used to maintain the cell temperature on small systems.
In order for the SOFC to be efficient, an effective partial oxidation catalyst element is required to convert the hydrocarbon fuels to hydrogen, which is capable of operating for long durations and undergoing a large number of thermal cycles at high temperatures. Various methods have been described for the preparation of catalysts. However, many of these methods involve costly use of precious metals.
Suda et al., U.S. Pat. No. 5,618,772 describe a method for producing a catalyst having high catalytic activity. Ultrafine alumina particles are mixed with a catalytic component and a substance for inhibiting the sintering of fine alumina particles, such as silica particles, to form a slurry mixture. This slurry mixture is dried and then the mixture is calcined to obtain a porous catalyst. Particles (50 wt. % or more) have a size of 100 nm (0.1 micron or less).
Clavenna et al., U.S. Pat. No. 5,395,813, describe a precalcined low silica content zirconia, especially one stabilized with yttria, as useful as a catalyst support or as a heat transfer solids component for conducting chemical reactions at high temperature. Desirably average particle diameter of both zirconia particles, and any catalyst component particles used in conjunction with the zirconia, ranges from about 30 microns to about 150 microns. However, such large particle sizes can lead to extremely bad adhesion when the catalyst material is applied to the substrate by washcoat methods.
Lahn et al, U.S. Pat. No. 5,160,456, describe a process for producing synthesis gas including hydrogen and carbon monoxide in a fluid-bed or spouted-bed process by reacting methane or a lower alkane with steam and oxygen in the presence of essentially non-catalytic, heat carrying solids with periodic addition of a steam-reforming catalyst. The heat carrying materials, such as alumina, have a mean particle diameter ranging from about 20 to 150 microns, preferably 30-150 microns, more preferably 30-120 microns. The catalyst may have a similar particle size distribution as that found in the heat carrying material or it may have a somewhat larger particle size, e.g., from 70-250 microns or larger. However, as stated previously, such large particle sizes can lead to extremely bad adhesion when the catalyst material is applied to the substrate by washcoat methods. The process of Lahn can also produce product with relatively high levels of methane. This is undesirable because, for use in combination with solid oxide fuel cell technology, it is desirable to have a level of methane no higher than 1%.
Pohl, U.S. Pat. No. 6,080,699, describes heterogeneous massive catalyst which includes at least one catalytically active component in the form of solid particles and at least one catalytically inert component in the form of solid particles wherein the components are dispersed in one another. The inert component has a mean particle diameter greater than the mean particle diameter of the catalytically active component, and the particles of the catalytically active material are grown on the particles of the catalytically inert material. However, the methods of Pohl are directed towards hydrogenation of natural fats and oils and are not typically useful for the preparation of a catalyst for fuel reforming.
K. Arnby et al. in J. Catal. 221, 252 (2004) and also in J. Catal., 223, 176 (2005) disclose a catalyst formulation useful for carbon monoxide oxidation involving a platinum catalyst distributed in locally high concentrations on a gamma alumina support. However, the reaction reported is not typical of fuel reforming reactions, as it yields an undesired product, carbon dioxide. The catalyst is reported to suffer significant thermal degradation over a time scale of 20 minutes and the advantages of this formulation are not found for oxidation of hydrocarbons, only for the oxidation of carbon monoxide.
R. J. Berger et al., Chem. Eng. Sci., 57, 4921 (2002) and Chem. Eng. Sci. J., 90, 173 (2002), describe the catalytic performance of a reaction bed dilution wherein catalyst particles are diluted with inert particles. It is reported that, in certain cases, such bed dilution may have an adverse effect on reactant conversion.
However, none of the described methods are suitable for preparing a reformate product that can be used in conjunction with a fuel cell in an economical manner. The methods reported above do not describe how to prepare a catalyst suitable for fuel reforming that will have enhanced thermal stability and allow one to reduce the level of the catalytically active metal while maintaining catalytic performance and provide means to effectively washcoat a catalyst carrier without subsequent loss of adhesion.
Fuel reforming is the process of reacting hydrocarbons, including for example, methane, natural gas, gasoline, kerosene, diesel fuel, and gas-oil mixtures, with an oxygen carrier, such as air, purified air, oxygen, steam, water, or carbon dioxide, over a catalyst to produce a reformate product consisting of primarily H2 and CO, with minimal amounts of residual hydrocarbons. To be used effectively with a solid oxide fuel cell, the reforming process must be efficient and produce material having a relatively low content of hydrocarbons; for instance, no more than 1 mole % methane and no more than 0.2 mole % other hydrocarbons is desirable in some solid oxide fuel cell operations. Some current catalysts are capable of processing the reaction of hydrocarbons with an oxygen carrier to sufficient conversion to meet reformate product qualities. However, these current catalysts are prone to deactivation, the primary mechanism for this is thermal degradation brought on by the high temperatures that these catalysts typically operate at, and in addition some applications call for a large (greater than 100,000) number of thermal cycles, including exposing the catalyst to periods of lean operation, which further contributes to thermal degradation.
Current means to overcome thermal degradation and improve durability include increasing the catalyst's metal content, employing a large volume of catalyst, or using complex catalyst support materials. Each of these approaches adds significant cost to the application. Thus, there is a need for an improved method for fuel reforming by catalytic partial oxidation and a need for a reformate catalyst element having improved durability and lower cost.